In the information age, present and beyond, explosive needs to communicate globally are causing accelerated growth in the deployment of communication satellites in the Low Earth Orbit(LEO), Medium Earth Orbit(MEO) as well as in the Geosynchronous Orbits(GEO). The need to constantly track and communicate with increasingly larger number of satellites by earth stations, stationary as well as mobile, demands development of efficient affordable phased array antennas capable of hemispherical or wider coverage scanning the entire sky and tracking multiple satellites both LEO, MEO and GEO, and continuously communicating two ways at multiple frequency bands. Invention of a new kind of affordable conformal phased array antenna capable of multiple beam, multiple band, multiple satellite communications is described here.
There exists a special equatorial circular orbit around the earth at the height of 22,300 miles, known as the geosynchronous or geostationary orbit, whose time period is equal to that of the rotation of the earth around its own axis. Therefore, an artificial satellite placed in that orbit will appear stationary from the surface of the earth.
Accordingly, the satellites in this special orbit do not have to be tracked and the focus of attention so far has been mainly directed to efficient and reliable communications with satellites in this special orbit. Because this orbit is 22,300 miles from earth, considerably large amount of transmit power have to be expended from the earth station as well as from the satellite to establish and maintain the two-way communication links.
Research, development and deployment of phased array antennas have received tremendous amounts of attention in the past fifty years in connection with radar systems and communication needs for mostly defense and some commercial applications. A phased array antenna is an array of antenna elements connected with a feed structure that energizes each element of the array with electromagnetic signals of appropriate amplitude and phase so as to form a beam in a chosen direction or a plurality of such beams in multiple directions so as to provide one-way or two-way communications. Hemispherical or wider communication coverage by phased array antenna systems has been realized by means of a multiple of planar phased array antennas placed on the faces of a pyramid or frustum of a pyramid with each planar phased array providing communication coverage to a segment of the hemisphere. The minimum number of required phased arrays is three whereas five planar arrays are often considered to be optimum. (For example, see G. H. Knittel, "Choosing the Number of Faces of a Phased Array Antenna for Hemisphere Scan Coverage", IEEE Transactions on Antennas and Propagation, Vol. AP-13, No. 11, November 1965, pp. 878-882). Because of the fact that every antenna element of the planar phased array has to be provided with a transmit unit and a receive unit with their associated control electronics containing amplifiers and other signal processing capabilities for simultaneous transmission and reception of electromagnetic beams, the phased array antenna systems are very expensive to manufacture and deploy. It is the prohibitive cost alone that has so far kept the phased array antenna system from playing its well deserved important role in the commercial arena.
However, driven by the defense needs of the past fifty years, many useful, albeit costly phased array antenna systems that are capable of providing hemispherical communication coverage have been proposed, designed, built, experimentally tested and deployed.
In U.S. Pat. No. 3,755,815, Stangel and Valentino described a Dome Lens phased array antenna system where in a planar phased array is capable of scanning the entire hemisphere or more by means of a feed-through hemispherical dome lens of antenna elements with fixed phase shifters, placed over the planar phased array. In this system, the planar phased array antenna in the diametric base plane, by itself, is capable of providing electronic scan coverage over approximately a conical space 60.degree. off-broadside. The dome lens placed over it then, by the principle of refraction, should be able to stretch that coverage over the entire hemisphere. However, such an array system will have significant beam distortions at lower elevation angles with associated polarization coupling and severe impedance matching and power loss problems. The system is costly to build and because of the lack of modularity in construction, fault repair will be expensive.
In U.S. Pat. No. 4,458,249 Valentino and Stangel described another lens array system for hemispherical coverage. In that dual lens array system, the first microwave lens is a three dimensional focal ring bootlace lens that is a figure of revolution as opposed to a planar phased array in their earlier patent mentioned above. This lens array is covered by a non-planar lens array dome or dielectric dome cover with an elevation angle dependent refractive index profile to provide the refractive effect necessary to extend the coverage to the full hemisphere and beyond. This dual lens system is electromagnetically coupled to a feed array matrix capable of providing a plurality of beam ports and in one embodiment may comprise electromagnetic horn antennas. The resulting total antenna system is very costly to build specially where large antenna gain at lower microwave frequencies(1 GHz--10 GHz) is required. Because proximity coupling of three subsystems are involved, impedance matching of the three systems through enclosed structures is a formidable experimental task and undesirable polarization coupling will be difficult to control. Also, because of the lack of modularity, fault diagnostics and corrections are more invasive and difficult.
In U.S. Pat. No. 5,543,811, Chethik have described a phased array system for hemispherical coverage by means of three large planar phased arrays placed on the three faces of a triangular pyramid wherein the base and height of the pyramid are chosen such that the antenna system provides higher gains at lower elevation angles to compensate for additional propagation loss and rain-related losses suffered in the commercially used frequency band of 20 GHz. Although there is nothing new disclosed in terms of the phased array antenna itself and the means of covering the hemispherical communication space, what is new are the details about the phased array antenna placement and orientations with respect to the horizon so that the specific loss compensation related needs at the desired frequency range can be correctly addressed.
One of the important operational requirements of the phased array antenna systems for satellite communications is multi-beam formation for multi-satellite tracking and communications. For satellite based phased array antenna systems, Sreenivas in U.S. Pat. No. 5,821,908 describes a phased array antenna which uses a spherical lens for microwave beam collimation. Using multiple spherical lenses and multiple phased array feeds, multiple beams could thereby be generated. However, as Valentino and Stangel had already pointed out in their U.S. Pat. No. 4,458,249, Sreenivas's system is not suitable for hemispherical coverage because of aperture blockage. Furthermore, at the lower end of the microwave frequency bands where the dimensions are relatively large, in the order of meters for accurate satellite tracking and communication, fabricating a spherical lens with radially variable dielectric constants will be very expensive.
The above discussion leads to the observation that there is a need for a low cost phased array antenna system, in the earth stations for satellite communications, capable of providing hemispherical or wider coverage. In order to reduce life cycle costs, there is a need for the phased array antenna to be modular in construction so that fault detections and corrections could be done easily by simple replacement of the bad module with a new good one. It is the primary objective of this invention to address these needs.
It is to be appreciated that the relative ability of the phased array antenna output power to form a beam in a given direction compared to every other direction with reference to the input power, defined as the antenna gain, makes narrower beam widths possible. However, narrower beam widths require larger arrays and the Transmit/Receive Units associated with each antenna element constitute approximately 40% of the Phased Array Antenna fabrication cost. The reduction of numbers of required T/R units by means of grouping the basic antenna elements is an important aspect of the low cost design that needs to be considered.
Advantages of small angle scanning afforded by a direction dependent phased array constituted with swichable element units and subarrays, are needed to be considered to reduce the cost of construction and operation of phased array antenna systems.
Phased array antennas with omni-directional scanning capabilities that can be deployed as a satellite in the low earth orbit to observe important events like ballistic missile launches and tracking, and have simultaneous communications with earth stations as well as other satellites in various kinds of orbits and altitudes are of great importance as well.
It is the main objective of the present invention to create a low cost phased array antenna architecture that will provide communication coverage over the entire hemisphere. Embedded in this primary objective is the need to reduce the number of transmit/receive units required by the phased array by grouping basic antenna elements of the array and forming clusters to which the T/R units are to be connected. It is another important objective of the present invention to do away with curved lines and surfaces in the construction of the phased array antenna so as to reduce the cost of construction. Still another important objective of the present invention is to create a highly modular phased array antenna structure so that failures and damages in the system can be easily detected, removed, and replaced more easily so that down time and life cycle costs are reduced substantially.